Technical Field
Embodiments of the invention relate generally to internal combustion engines, and more particularly to turbocharged engines. Other embodiments relate to diesel engines and other engines with multi-stage turbocharger arrangements.
Discussion of Art
Large ships such as container ships, tankers, bulk carriers, and cruise ships typically carry two types of diesel engines: main propulsion engines and auxiliary engines. The main propulsion engines on most large ships are “Category 3” marine diesel engines, which can stand over three stories tall and run the length of two school buses. Auxiliary engines on large ships typically range in size from small portable generators to locomotive-size engines.
Category 3 engines generate significant emissions of PM 2.5 (diesel soot), SO2 (sulfur dioxide), and NOx (nitrous oxides). All these substances are recognized as pollutants by some governmental agencies. For example, the U.S. Environmental Protection Agency (EPA) estimated that, in 2009, Category 3 vessels contributed almost 913,000 tons (10 percent) to the U.S. national mobile source NOx inventory, about 71,000 tons (24 percent) to the mobile source diesel PM 2.5 inventory, and nearly 597,000 tons (80 percent) to the mobile source SO2 inventory. Accordingly, the EPA adopted standards that will apply to Category 3 (C3) engines installed on U.S. vessels and to marine diesel fuels produced and distributed in the United States. The adopted standards include limitations on NOx emissions.
NOx emissions vary according to engine speed, combustion temperature, and the quantity of fuel burned. Lower speeds and higher combustion temperatures tend to raise the amount of NOx produced per quantity of fuel burned. As will be appreciated, low speed and high combustion temperature can be desirable characteristics of high-load internal combustion engines such as marine diesel propulsion engines. Therefore, the simplest path for reducing NOx emissions is to reduce the quantity of fuel burned per unit of useful power.
Turbochargers can be useful for enhancing fuel efficiency of piston-cylinder engines, by augmenting the mass and pressure of air inhaled into each cylinder during its intake stroke. However, turbochargers for large (e.g., Category 3) engines can be difficult to fit into the machinery space provided for the engines and their interfacing equipment.
As will be appreciated, it is desirable to minimize the machinery space within large ships, which are built to carry cargo or otherwise. Therefore, it is desirable to provide a marine diesel turbocharger arrangement that satisfies forthcoming EPA or other governmental standards while not requiring excessive machinery space.